This invention relates to a coupling element for realizing an electromagnetic coupling and to an apparatus for coupling a radio communication device, advantageously a cellular mobile phone, to an external antenna. The invention can be advantageously applied for coupling a mobile phone to a car antenna.
A common way of improving the car use characteristics of a mobile phone is to install outside the body of the car a so-called car antenna to which the mobile phone is connected via a coaxial conductor and a RF connector at the end of the conductor. Therefore, a prior art mobile phone must include a counterpart for said RF connector. Then the antenna of the mobile phone must be made inoperative either by an automatic switch connected to said counterpart for the RF connector or by the user with a separate switch, lest simultaneous operation of the both antennas cause interference and losses.
Said RF connector and its counterpart may constitute a standard coaxial interface based on galvanic contact, but such a solution is susceptible to oxidization, dirt and wear. A solution is known from Finnish Patent No. 84536 and corresponding EPO application EP-0 399 975, wherein the RF coupling between the mobile phone and the connector part in the car installation set is made capacitively by means of metal plate counterparts. This arrangement avoids said oxidization, dirt and wear problems, but it does not eliminate the need for a separate switch to disconnect the mobile phone antenna for the duration of car use. In addition, the switching means and the transmission lines connected to the means require space inside the mobile phone.
There also exist prior art methods and devices for electromagnetically coupling a car antenna to a mobile phone antenna. The simplest electromagnetic coupler is an induction loop placed around the mobile phone antenna. Patent document GB-2 266 997 discloses a solution according to FIG. 1 of this application, wherein a connector part B comprising a resonator element C is attached with adhesive tape or the like to the casing of a mobile phone A to be used in a car. By attaching the connector part B to the mobile phone A said resonator element C is brought so close to the mobile phone antenna D that it is electromagnetically coupled to the antenna and starts to resonate, whereby RF power radiated by the mobile phone antenna is coupled to it and that power is further taken via a coaxial conductor E to the car antenna. A disadvantage of this arrangement is that the coupling between the mobile phone antenna D and the resonator element C depends on their positions relative to each other. An external resonator element also puts an electric load on the mobile phone antenna, thereby decreasing its resonating frequency and weakening the coupling in the upper part of the frequency band used.